Side rails can be built only occasionally along the line. With a modern automated system these could be used to divert trains around the section of track with a disabled train, there's not a huge additional cost to do this.
The most expensive part of the monorail system (not counting the trains themselves) would be the equipment to transfer from one rail to another. So if you don't count that the cost isn't huge.
Yes people have certainly made decisions not to visit a park based on transportation. People may not cancel a trip over it but if leaving MK decide they want to park hop for a few hours odds are much greater they're going to Epcot. We also certainly know the "monorail resorts" as they are referred to are the top choice for many people.
Once they have already used that day ticket, I don't think Disney cares whether or not they go to any specific park or not. Besides the alternative methods are not all that difficult to deal with if you want to go someplace bad enough. That is just a whim decision done for convenience. If they made an ADR then they would find the way there.
Yes mechanical problems are always going to happen, but they should be extremely rare. I agree if we're basing things off Disney's recent track record monorails don't look like a good option. It's better to have half your busses running rather than multiple trains being towed.
Although I have never witnessed it personally, from what I hear there is more of a problem now then there was before, but, even new equipment can have problems. If they are going to spend money, I rather they spend it on a newer Monorail fleet to replace what is already there, then to spend it on something that will not be used as much as you might think. When you figure the total number of miles compared to the number of breakdowns, they are a long way away yet of not being listed as dependable. Again, I have never had a problem using the Monorails even in recent years. Maybe I'm just lucky.
Again if your going to discuss the cost of buses vs. monorail be sure to include the recurring per year labor cost. At almost $100,000 per year per bus that adds up pretty quick.
I should have been more specific by bus loop I meant the series of bus stops located at the MK. If a bus breaks down at the entry/exit or traffic backs up at that point all bus traffic to and from the MK has to stop. It's happened before and it can be a real mess.
I do not have access to what it costs to run that Monorail all day long, but, I'd bet that there is a huge crew that works the monorail shops to keep them going constantly. What we also tend to overlook is the crowds we see on the Monorail are limited to certain times of the day. They rest of the time they are running practically empty. Parts are universal on buses, on Monorails they are specific to the specifications of the buyer. They cost a lot more to maintain and I'd hate to have to pay the electric bill to operate them.
What this all boils down to, is that the Monorails exist today because when they built the park, it seemed like it would absolutely be the transportation of the future. It isn't and there is a reason for that. It still isn't economical enough to warrant that type of investment. If it were, all the cities would have them. At this point, just like they are in WDW, they are a novelty. It is a service vehicle to transport people to MK and Epcot, but, it is a very thin need. Mostly it is there because it always has been there.